At present, electrolyte materials used in the lithium ion battery industry are mainly multi-solvent systems of cyclic carbonates and linear carbonates compounds, and LiPF6 lithium salts. The operating mode of the electrolyte system is still exposed to safety risks from technical perspective, which is mainly due to the high volatility and inflammability of the carbonates electrolyte materials. In the application field of hybrid electric vehicles and all-electric vehicles with high requirements for high safety, large capacity, and high rate discharge, the safety issue is an important factor restricting the application of these materials. Therefore, a new generation of safe, effective, and environmentally-friendly organic electrolyte materials are being proactively developed both in China and abroad.
A substance, in the form of liquid at or near room temperature, composed of ions is referred to as the room-temperature ionic liquid, room-temperature molten salt, organic ionic liquid or the like, which, however, tends to be called as ionic liquid. Since such ionic liquid has low volatility and low flammability, better thermal stability, good chemical and electrochemical stability, the ionic liquid gains wide application prospect in such fields as green chemistry, industrial catalysis, industrial solvents or the like. Due to such characteristics as high safety and high electrochemical stability, researches have been proactively conducted in terms of using ionic liquid as an electrolyte solution of a lithium ion battery.
The ionic liquid electrolyte materials for use in the lithium ion batteries can be categorized into two types: one is a molten lithium salt in the form of ionic liquid, and the other is a molten lithium salt in the form of ionic liquid added with a corresponding additive. The first generation of ionic liquid is an organic molten salt with AlCl4 as anions. Such ionic liquid is apt to be hydrolyzed, and reacted with water to give HCl. Therefore, no further study is conducted for the application of the first generation of ionic liquid in the lithium ion batteries. The second generation of ionic liquid is an organic room-temperature molten salt having the imidazole cations as the positive ions and fluorinated inorganic or organic anions as the negative ions. Such ionic liquid exhibits poor electrochemical reduction stability, and thus is not considered having commercial application prospects in the high performance batteries. The third generation of ionic liquid employs the non-imidazoles cations and fluorinated inorganic or organic anions. At present, the most suitable ionic liquid for the lithium ion batteries is N,N-dialkyl piperidine (Patent JP2006260952). However, these ionic liquids cause a greatly reduction of the output power of the lithium ion batteries, because these ionic liquids having high chemical stability have higher viscosity, which compared with the traditional carbonates electrolyte, causes a significant reduction of the conduction velocity of the lithium ions (O. Borodin et. al. J. of Physical Chemistry B, 2006, 10(34), pp. 16879-16886). Compared with the traditional carbonates-based electrolyte lithium ion batteries, the ionic liquid-matrix electrolyte lithium ion batteries still have low output power and small charge capacity. Lee et al. (Electrochem. Comm. 8 (2006) 460) have reported that using the imidazoles ionic liquid with an ester radical on the N atom as the electrolyte of the lithium ion battery improves the conductivity and diffusion velocity of the lithium ions. However, these imidazoles ionic liquids are poor in terms of electrochemical stability. R. West et al. have reported in the U.S. Pat. No. 7,679,884B2 and US2009088583-A1 that silicon-based quaternary phosphonium and silicon-based quaternary ammonium ionic liquids. These ionic liquids exhibit improved electrochemical stability but still have high viscosity.
The choline chloride plays an important role in the cells function, and biosynthesis and degradation of the choline chloride control life activities of the cells. The choline chloride has excellent biocompatibility and is biodegradable. In addition, the choline chloride, as a feed additive or the like, can be industrially manufactured and is thus a very cheap raw material. Further, choline hydroxide has been used as a basic catalyst for the Aldol condensation reaction. The choline derivatives having low melting point are the focus of a variety of researches. Some choline analogs have been successfully synthesized [Pernak, Chemistry—A European Journal, 2007, 13(24), pp. 6817-6827]. [Me3NC2H4Y][Cl] (Y═OH, Cl, OC(O)Me, OC(O)Ph) and MCl2 (M=Zn, Sn) is a viscous liquid which is conductive around room temperature, and typically used for electro-deposition. However, choline chloride-based ionic liquid has not been used as an electrolyte or an additive for the lithium ion battery.